Optical recording elements on which a projected laser beam from a semi-conductor laser or other devices perform the recording/retrieval/erasing of information, have attracted attention as high-density recording mediums have been developed with large storage capacity and high portability. A magneto-optical recording element using a magnetic film as a recording medium is the most developed device in rewritable optical recording elements. A magneto-optical disk comprises a recording film containing a magnetic film having vertical magnetic anisotropy formed on a transparent substrate in which information is recorded by reversing the direction of the magnetization of the recording film. In an optical memory system for such a magneto-optical recording element, the recording and erasing of information are performed by changing the condition of the magnetization of the recording film with an external magnetic field applied thereto while the temperature of the recording film is raised by a laser beam, or by the demagnetizing field of the magnetic film itself. On the other hand, the retrieval of information is performed by utilizing the effect that when the laser beam incident on the recording medium is reflected, the plane of polarization of the beam is rotated according to the direction of the magnetization.
Such an optical memory system verifies whether information is properly recorded on the element when new information is recorded in an area of the optical recording element (optical disk) where no information is stored therein. In the use of a conventional optical memory system, the aforementioned check cannot be performed until the optical disk rotates one time and the recording area thereof returns to the position opposing the optical head (a first rotation) after the completion of information recording.
In the case of recording different information in an area where information has been previously recorded, the following steps are taken in order to check whether the recording operation has been correctly performed: (i) the previous information stored in the recording area is erased by setting the magnetization of the recording film in the initial direction; (ii) the optical disk rotates a single revolution so that the recording area returns to the position opposing to the optical head (a first rotation); (iii) the new information is recorded in the recording area; (iv) the optical disk rotates another single revolution so that the recording area returns to the aforesaid position (a second rotation); and (v) the new information is read out.
As described above, in order to confirm whether the recording of information has been correctly performed, at least the time for one rotation of the optical disk is required in the former case and at least the time for two rotations of the optical disk is required in the latter case. This brings about a considerable increase in the time required for information recording and verification.
In recent years, the light beams of a semi-conductor laser or the like have been utilized for performing the recording/retrieval/erasing of information on such an optical recording element. A lens system for converging a light beam from the semi-conductor so as to irradiate the optical recording element and an optical system (e.g. optical pick-up device) comprising a photodetector for detecting the luminous energy of reflected light from the optical recording element are required to be two-dimentionally moved in relation to the optical recording element at a high speed. Therefore, it is quite difficult to accurately and precisely position the optical system in relation to the optical recording element.
In a conventional method, while rotating the disk-shaped optical recording element, the optical system is one-dimentionally moved in the radial direction of the optical recording element, thereby recording information on the disk surface of the optical recording element, and reading or erasing the information recorded therein.
Generally, such a disk-shaped optical recording element has, as shown in FIG. 22, a number of guiding grooves 80 disposed on one surface of a substrate 79 for guiding a converged light beam 82, and a recording film 81 on the same surface provided with the guiding grooves 80. The recording/retrieval/erasing of information are performed by directing the light beam 82 converged by an objective lens 83 from the other surface of the substrate 79 onto recording grooves 84.
The guiding grooves 80 are disposed in large numbers on the substrate 79 for the purpose of performing the accurate positioning of the converged light beam so as to record information in a desired area or read out the same from a desired area. As shown in FIG. 23, it is common in such an arrangement that a sequence of pits 85 are interposed in each of the recording grooves 84 at a part and the address of the recording groove 84 is indicated by the lengths and positions of the pits 85. More specifically, a recording film is formed in a continuous form on a transparent substrate whose surface is uneven with guiding grooves and address pits. These grooves and pits are formed by surface portions of the substrate.
If the power of a semi-conductor laser used for recording information is unexpectedly high (in fact, recording power varies depending on an optical memory system), it often happens in the above optical recording element that a recording bit 86 recorded on the recording film over the portion of the recording groove 84 extends to the recording film constituting the adjacent guiding grooves 80 as shown in FIG. 24. As a result, a signal, which should not be read out, is intermixed with a correct signal when reading a signal from either of the adjacent guiding grooves 80 used the light beam. This results in the occurrence of crosstalk.
Such deformation of recording bits is seen in the direction of the recording grooves. That is, the sizes of recording bits vary in the direction of the recording grooves due to variation in recording power resulting in deterioration of signal quality.
A recording bit in an optical memory system used for the optical recording element such as an optical disk and magneto-optical disk has a very small area, i.e. approximately 1 .mu.m.sup.2, so that accurate and precise control is required in the accessing operation of light beam. In the field of an optical disk for data recording, two types of tracking methods employed; a continuous groove method and a sampling method.
In the continuous groove method, a groove 92 is disposed on a substrate 91 as shown in FIG. 25 and the diffraction of a light beam at the groove 92 is utilized for detecting dislocation of the light beam.
More specifically, a light beam from a laser beam source (not shown) is irradiated on the groove 92 formed on the substrate 91 through a half mirror 93 and objective lens 94. A reflected light from the groove 92 is incident on a two-element photodetector 95 after passing through the objective lens 94 and half mirror 93. The differential between signals respectively issued from photodetecting sections 95a and 95b in the two-separate photodetector 95 is amplified by a differential amplifier 96, thereby generating a tracking error detection signal.
In the sampling method, as shown in FIG. 26, a substrate 97 is provided thereon with a pair of pits 98 and 99 for detecting tracking errors, which are spaced in a perpendicular direction to the track, with the center line of the track indicated by arrow B therebetween. These bits are positioned equidistant from the center line B on which data pits 100 are aligned. The amplitude of read out signals respectively issued from the pits 98 and 99 are compared, and if the amplitude S.sub.1 ' of the read out signal from the pit 98 is greater than the amplitude S.sub.2 ' of the read out signal from the pit 99 (see FIG. 27(a)), the light beam is deemed to be closer to the pit 98 along the direction indicated by arrow A. If the amplitude S.sub.1 and S.sub.2 of the read out signals from the pits 98 and 99 are equivalent (see FIG. 27(b)), the light beam is deemed to be in the center of the track along the direction indicated by arrow B, and if the amplitude S.sub.1 " of the read out signal from the pit 99 is greater than the amplitude S.sub.1 " of the read out signal from the pit 98, the light beam is in a position closer to the pit 99 along the direction indicated by arrow C. A pair of pits 98 and 99 are disposed in large numbers on the track.
In the sampling method described above, reflected lights from the pits 98 and 99 disposed on the substrate 97 successively pass through the objective lens 94 and half mirror 93, and then are incident on the photodetector 101, as shown in FIG. 28. Thereafter, signals generated in accordance with the amplitude of the read out signals from the pits 98 and 99 are transmitted in succession from the photodetector 101 to a shift register 103 through a waveform shaping circuit 102. The differential between the read out signals of the pits 98 and 99 is obtained according to a signal from a timing signal generator 104 and amplified by the differential amplifier 105 in order to issue a tracking error detection signal.
In the above continuous groove method as shown in FIG. 25, even if the light beam accesses the appropriate position, i.e. the center of the track, the tracking error detection signal does not become "0" when the substrate 91 is inclined. Therefore, a problem exists in that it is difficult to judge whether the tracking error detection signal does not become "0" because of a tracking error or because of the inclined substrate 91. In order to overcome the above problem, incline of the substrate 91 has to be limited to a small allowable range in this method.
Although such a tracking error detection signal caused by the incline of the substrate is hardly generated in the sampling method shown in FIG. 26 to FIG. 28, the sizes of the pits 98 and 99 are required to be accurate in width and depth in order to perform the accurate detection of tracking errors. Since the detection of a read out signal is performed by sampling in this method, the time at which the light beam passes through the pits 98 and 99 is determined beforehand. This requires the accurate positioning of the pair of pits 98 and 99, in relation to another pair of pits 98 and 99, causing an increase in the production cost of the substrate 97.